After you unlock Mario, go through the doors that lead to the basement, and follow the big boo into the courtyard opposite the stairs. Once there, destroy the biggest boo and enter the cage that falls out. Then beat the first mission and when you do the second mission, go up the stairs to the right and go in the first door on the right. If you have unlocked the red boxes, hit it and get the flower, making mario float. float to the top of the room, and go through the door on the platform. In the room is a picture of Luigi, enter the picture and beat the level and King Boo, and you get Luigi's key. Use it to unlock the door with an L above it next to Mario's door in Princess Peach's room on the first floor.